As noted in Chapter 4, a number of aircraft failures resulting from fatigue crack growth from preexisting flaws or defects, which were introduced during material processing or manufacturing, caused the Air Force to extensively revise their Aircraft Structural Integrity Program (ASIP) in the early 1970s to include damage tolerance requirements. These requirements were defined in MIL-A-83444 (DOD, 1987) and MIL-STD-1530A (DOD, 1988) and were incorporated into the designs of the new aircraft then under way (e.g., the B-1A, F-16, and A-10). However, to protect the structural safety and assess the durability of the vast majority of Air Force aircraft that were not designed to these requirements, the Air Force and the aircraft contractors performed durability and damage tolerance assessments (DADTAs) on the aircraft models that were already in the operational inventory. By the early 1980s DADTAs had been performed on the F-4C/D/E, A-7D, C-5A, C-141, F-111, B-52D, E-3A, F-5E, T-38, T-37/A-37, KC-135, SR-71, T-39, KC-10, C-130, and F-15. Also, because of changes in use conditions, the durability and damage tolerance of both the A-10 and F-16 had to be revisited after only a short time in operational service.
From the standpoint of safety, the most important outputs from these assessments were the identification of fatigue-critical areas, the determination of safety limits for these areas, and the development of safety inspection requirements. In addition, for some of the larger transport aircraft, estimates were made of the onset of widespread fatigue damage (WFD) and risk analyses were performed (e.g., on the C-5A, KC-135, and C-141). Where appropriate, lower-bound estimates were made of the major component modification or replacement times and modification options were defined.
The overall approach or methodology used in conducting the DADTAs is illustrated in Figure 5-1. As can be seen in this figure, the four primary tasks in the assessments are (1) the identification of fracture-critical areas;1 (2) the development of the operational stress spectra for these areas; (3) an assessment of initial flaw distributions and/or the maximum probable initial flaw sizes; and (4) the determination of the safety limits, inspection intervals, and, for fail-safe designs, the estimated onset of WFD. The results were then used to update the individual aircraft tracking programs and the force structural maintenance plans for the aircraft, both of which are key elements of ASIP.
To obtain improved visibility of the actions that will be necessary to protect the structural safety of the Air Force's aging aircraft listed in Table 3-1 throughout their projected operational lives and to obtain the best estimates as to when the aircraft will likely be facing the economic impacts of major modifications or replacements, the committee strongly recommends that the DADTAs of these aircraft be updated periodically. In general, an update about every five years is appropriate.
The urgency to perform these updates varies among the different aging aircraft types, depending on several factors: (1) whether the aircraft structure is designed to be fail-safe or is largely of a single load-path design, where missing a critical area could lead to the loss of an aircraft; (2) whether a replacement aircraft has been identified and the older aircraft are being phased out of the inventory; (3) the extent and nature of fatigue cracking problems the aircraft are currently encountering; and (4) whether there has been a recent independent review of the aircraft and corrective actions are already under way. Table 5-1 summarizes these different factors for each of the Air Force's aging aircraft types shown previously in Table 3-1. Also shown in Table 5-1 is the committee's assessment of the priority that should be assigned to performing the DADTA update for each type of aircraft. Those of greatest concern, based on the highest potential for structural safety problems, were given a number 1 priority and those with the least immediate concern were given a number 3 priority. However, it is recommended that the DADTA update be performed on all of the aircraft within the next five years and updated at approximately five-year intervals.
The committee recognizes that the level of effort involved in performing these updates will vary significantly between the different types of aircraft as a function of aircraft complexity, variations in use, the numbers and types of cracking